Computer archive systems include computer systems that store documents (e.g., immutable documents). These documents are also referred to herein as files and can include any form of stored data. An archive system comprises one or more separate computers having specialized archive software and access to a large amount of storage space (e.g., hard drives, magnetic tapes). Archive systems are often owned or operated by a party that provides storage space and related services to clients. During typical operation of an archive system, a client acquires a restricted account on the system to allow for storage and retrieval of electronic documents. Archive systems facilitate retrieval of such stored documents by utilizing document identification codes. For example, when presented with a document by a client, certain types of computer archive systems produce a short and unique document identification code (document ID) that refers to that particular document.
After a document ID is assigned, an archive system operator or client can typically retrieve that document from the computer archive system at any time before it has been deleted by requesting that document ID. Whether a requested document is on disk or on tape, a typical archive system is adapted to locate it and retrieve a copy. However, archive systems frequently fail to properly maintain documents and document copies. Indeed, equipment and equipment operators are subject to failure or inadequate performance. For example, typical archive systems create potential for error by periodically copying documents from hard drive storage space to other storage media (e.g., disk, tape) to improve cost efficiency. Further, such storage media are handled within the archive system by a robot system, which introduces more potential for error in the retrieval of thousands of storage media. Situations occur in which some documents are lost, damaged, overwritten, and so forth. In certain situations, unscrupulous individuals can attempt to compromise archive security by attempting to directly or indirectly seek the destruction or corruption of archived information. For example, under some circumstances (e.g. embezzlement), other parties can attempt to bribe the archive operator to purposely delete particular documents. Accordingly, clients of archive systems frequently do not trust their computer archive systems or their archive system operators and wish to have guarantees of data retention. Also, it is now recognized that it is desirable for clients to be able to delete certain documents from the archive without enabling the archive operators to make deletions at will.